Generally, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode each including a catalyst layer (electrode catalyst) and a gas diffusion layer (porous carbon) to form a membrane electrode assembly (MEA). The membrane electrode assembly is interposed between separators (bipolar plates). Normally, in use, predetermined numbers of the fuel cells are stacked together to form a fuel cell stack mounted in a vehicle, for example.
In the membrane electrode assembly, a thin solid polymer electrolyte membrane is used. Therefore, the solid polymer electrolyte membrane may be damaged undesirably due to the mechanical stress resulting from, e.g., the difference between pressures of reactant gases supplied to the solid polymer electrolyte membrane.
In this regard, for example, a fuel cell as disclosed in Japanese Laid-Open Patent Publication No. 2006-318940 is known. As shown in FIG. 7, the fuel cell includes a unit cell 1, and first and second separators 2, 3 sandwiching the unit cell 1. The unit cell 1 includes a cathode 5a, an anode 6a, and a solid polymer electrolyte membrane 4 interposed between the cathode 5a and the anode 6a. 
A first gas diffusion layer 5b is provided on the cathode 5a, and a second gas diffusion layer 6b is provided on the anode 6a. The surface area of the anode 6a and the surface area of the second gas diffusion layer 6b are smaller than the surface area of the solid polymer electrolyte membrane 4. The surface area of the cathode 5a and the surface area of the first gas diffusion layer 5b are smaller than the surface area of the anode 6a and the surface area of the second gas diffusion layer 6b. 
A first seal S1 is interposed between the first separator 2 and the solid polymer electrolyte membrane 4 around the cathode 5a. A second seal S2 is interposed between the first separator 2 and the second separator 3 around the anode 6a. 
In the unit cell 1, steps, buffers, gas inlets, and gas outlets are not used as power generation areas. In these portions, no gas diffusion function is required. However, the first gas diffusion layer 5b and the second gas diffusion layer 6b extend to a buffer, and the second gas diffusion layer 6b extends to a step.
Normally, the first and second gas diffusion layers 5b, 6b are porous layers of expensive material such porous carbon clothes and porous carbon papers. Therefore, the unit cell 1 is not economical, and the overall cost of the fuel cell is high.
Further, in an attempt to achieve the desired gas diffusion function, the first and second gas diffusion layers 5b, 6b are relatively thick. In the structure, it is difficult to achieve the sufficient flow field height in the buffers, the gas inlets, and the gas outlets. Thus, the pressure loss is increased, and the gas cannot be distributed smoothly to the flow fields at the electrodes.